1. Field of the Invention
Apparatuses consistent with the present invention relate to a differential amplifier and an active load for the same, and more particularly, to a differential amplifier and an active load providing a common mode feedback.
2. Description of the Related Art
A differential amplifier is one of the most important analog circuits. Most high speed analog circuits are currently embodied by using the differential amplifier. The differential amplifier functions to amplify the difference between two input voltages and generally includes two input terminals.
FIG. 1 is a circuit diagram of a differential amplifier according to the conventional art.
Referring to FIG. 1, the differential amplifier according to the conventional art includes input transistors MN1 and MN2, a current source Iss, load transistors MP1 and MP2, and resistances Rr.
An input signal Vin inputted in a gate of the input transistors MN1 and MN2 is amplified and outputted via an output terminal Vout. In this instance, the transistors MP1 and MP2 function as a load. Resistances Rr provide a common mode feedback to a differential amplifier and stably maintains a common mode bias point of the output terminal Vout. In this instance, a node P may be a virtual ground for a small signal.
The load transistors MP1 and MP2 function as load in the differential amplifier. Generally, when a passive device such as a resistance is not used, but rather an active device such as a transistor is used as a load, the load is an active load.
However, a voltage gain of a differential amplifier illustrated in FIG. 1 is only about gmn/gmp. In this instance, gmn is a transconductance of an NMOS transistor and gmp is a transconductance of a PMOS transistor. Also, a pole is positioned in a low frequency because of the resistance Rr, and the frequency characteristic becomes deteriorated. Accordingly, the differential amplifier may not be applicable to an application needing a high frequency gain and a wide bandwidth.
FIG. 2 is a circuit diagram of another differential amplifier according to the conventional art.
Referring to FIG. 2, the differential amplifier according to the conventional art includes input transistors MN1 and MN2, a current source Iss and two active load circuits 210 and 220.
The active load circuits 210 and 220 each include an NMOS transistor, a PMOS transistor, a capacitor and a current source. A bandwidth of the differential amplifier may be increased by using the active load circuits 210 and 220. Moreover, a gain of the differential amplifier may be improved by increasing an output resistance.
However, the differential amplifier illustrated in FIG. 2 has a problem in that the swing of an output is significantly reduced. If the swing of the output voltage is small, an applicable application is severely restricted. This limited output voltage swing may cause a serious malfunction in a low voltage application. Also, in the case of the differential amplifier illustrated in FIG. 2, a transconductance of a transistor varies according to a process variation in fabricating the transistor. A bandwidth and a gain also vary according to the process variation.
Accordingly, compared with the differential amplifier according to the conventional arts illustrated in FIGS. 1 and 2, a new differential amplifier and active load which can provide a higher gain, be used in a wider frequency band, and effectively compensate the change of a common mode operating voltage according to process variation is highly needed.